1. Field of the Invention:
This invention relates to an exposure system suitable for use in a copying machine to illuminate, in the pattern of a strip, the surface of an original document.
2. Description of the Prior Art:
In a conventional exposure device employed in an electronic copying machine to illuminate each original document, and illumination unit composed of a gutter-like mirror and an elongated light-source lamp extending along the length of the mirror is provided, whereby a narrow, elongated, strip-like section, which extends along the width of an associated table adapted to support each original document thereon, is illuminated by light reflected by the mirror and light radiated directly onto the strip-like section from the lamp. The thus-illuminated strip-like section is then caused to move along the length of the table so that an image of the whole area of the original document placed on the table is projected on a photosensitive member by way of an optical projection system.
In order to obtain a duplicate picture of mark of uniform density and good quality, it is essential to make the illuminance distribution uniform on the photosensitive member. For this purpose, it is necessary to employ a lamp as a light source in which dimensions of the light-emitting parts of the filament and the lengths of the light non-emitting sections between the adjacent light-emitting parts are suitably designed. Furthermore, certain additional means is also provided in an attempt to obtain a uniform illuminance distribution pattern. For example, a light-controlling blade is provided along an edge portion of the mirror in such a way that the blade can be pulled out from the edge portion. By adjusting the pulled-out length of the blade, the light is partly shielded to control the quantity of light which is allowed to reach the section to be illuminated. Alternatively, the reflection characteristics of the mirror are changed by suitable means. Even if a uniform illuminance distribution pattern is obtained by such means, this distribution pattern still contains small irregularity in light quantity, which is generally called "ripple". This ripple is attributed to the winding irregularity and deformation of the filament of each light-source lamp. In order to solve this ripple, it is effective to broaden the light to be irradiated onto the section. For this purpose, it may be contemplated to use a lamp equipped with a frosted glass envelope as a light-source lamp or impart light-diffusing function to the mirror. These approaches are however accompanied by such problems that the use of a lamp equipped with a glass envelope, which is frosted at its entire surface, leads to a lowered light-condensing efficiency and the light cannot be broaden to any sufficient extent if the diffusion function of the mirror is solely relied upon.